Remove referenced to dropped views

Summary:
There is currently a bug where we never release any Views that no longer display, still storing hard references in NativeViewHierarchyManager. This diff is fixing this bug, and here is how:

a) there is already logic in place to drop FlatShadowNodes (UIImplementation.removeShadowNode).
b) there is already logic in place to drop Views (NativeViewHierarchyManager.dropView(int reactTag) - used to private but needs to be protected so I can call it)
c) (the missing part) when we are about to drop a FlatShadowNode, check if it mount to a View (i.e. there is a View associated with that node), put it into a ArrayList. When we finished updates to a nodes hierarchy (which happens in non-UI thread), collect ids from those nodes and enqueue a UIOperation that will drop all the Views. We can either forward nodes as FlatShadowNode[], or only ids as int[]. Both should be fine, but as a rule of thumb we don't touch shadow node hierarchy from UI thread (as we don't touch Views from non-UI thread) so passing int[] is what this code is doing.

Reviewed By: ahmedre

Differential Revision: D2757310

ReactAndroid/src/main/java/com/facebook/react/flat/FlatNativeViewHierarchyManager.java

@@ -122,6 +122,12 @@ import com.facebook.react.uimanager.ViewManagerRegistry;
     resolveView(reactTag).setPadding(paddingLeft, paddingTop, paddingRight, paddingBottom);
   }
 
+  /* package */ void dropViews(int[] viewsToDrop) {
+    for (int viewToDrop : viewsToDrop) {
+      dropView(resolveView(viewToDrop));
+    }
+  }
+
   /* package */ void detachAllChildrenFromViews(int[] viewsToDetachAllChildrenFrom) {
     for (int viewTag : viewsToDetachAllChildrenFrom) {
       View view = resolveView(viewTag);